1. Field of the Invention
The present invention relates to an image forming apparatus for conducting image formation such as a copying machine, a printer, and a facsimile machine, and more particularly to an image forming apparatus that transfers a toner image formed on an image bearing member onto an intermediate transfer member and thereafter transfers the toner image on the intermediate transfer member onto a transfer material.
2. Related Background Art
Up to now, there has been known a four-color full color image forming apparatus using an intermediate transfer member. An image forming process will be described in brief. First, a toner image of a first color (yellow) is formed on a photosensitive member and the toner image is then primarily transferred onto the intermediate transfer member. Then, the primary transfer is sequentially conducted on the other three colors, that is, magenta, cyan and black, and the toner images of those four colors are superimposed on the intermediate transfer member. Thereafter, the toner images of the four colors are secondarily transferred in a lump onto a transfer material P such as a sheet by a secondary transfer member, thereby being capable of obtaining a full-color image.
By the way, in the image forming apparatus using the above conventional intermediate transfer member, the primary transfer of the toner image formed on the photosensitive member onto the intermediate transfer member is conducted four times on the yellow, magenta, cyan and black colors. However, there is a case in which at the time of transferring the magenta color of the second time, the yellow toner which has been already transferred onto the intermediate transfer member is returned to the photosensitive member from the intermediate transfer member in the primary transfer position. This is called "counter-transfer", and similarly, the yellow toner and the magenta toner, which have been already transferred onto the intermediate transfer member at the time of transferring the cyan color of the third time, are counter-transferred onto the photosensitive member, and the yellow toner, the magenta toner and the cyan toner which have been already transferred onto the intermediate transfer member at the time of transferring the black color of the fourth time, are counter-transferred onto the photosensitive member.
In the above manner, there arises such a problem that the density of the output image is lowered since the number of times at which the toner primarily transferred more precedently is counter-transferred, that is, the amount of counter-transfers of such a toner is more increased. That is, the color heterogeneity of the output image may occur. In particular, the counter-transfer is liable to occur at the photosensitive member having a photosensitive layer deteriorated by wear or the like, and also becomes remarkable under the circumstances where the temperature is high and the humidity is high.